Transparent Paths between Quaternary Volcanoes in Northeastern Japan Revealed from S-wave Envelope Analysis

S-wave envelopes of deep and intermediate depth microearthquakes observed at stations located in the back arc side of northeastern Japan are significantly different from that observed in the fore arc side. Apparent durations and peak delays of S-wave envelopes are relatively long at stations in the back arc side, while are not prominent in the fore arc side (Obara and Sato [1995]). In this study, we examine the path dependence of S-wave envelope of shallow(35km ∼100km) earthquakes which occurred along the subducting Pacific Plate to clarify the regional difference of medium heterogeneity. To characterize an S-wave envelope, we measure the peak delay time from the S onset t_p. By looking at the t_p obtained from the data having almost the same hypocentral distance (150km ∼200km) and the same earthquake magnitude, we find significant difference in their t_p. For example, at a station located near the Iwate volcano, t_p in 16Hz ∼32Hz band is 3.6s for a ray propagating beneath the volcano and that is as short as 1.7s for a ray passing far from the volcano. From the systematical examination of t_p for many different pairs of stations and events, we further find that t_ps are large for rays traversing beneath Quaternary volcanoes in northeastern Japan, while t_ps are small for the rays propagating in narrow paths between Quaternary volcanoes in depth down to about 40km. Our peak delay analysis revealed the spatial variation of medium heterogeneity and the existence of transparent and less scattering paths between Quaternary volcanoes in the back arc side.